Evaluating post-harvest practices for the quality and safety of Kona coffee

Coffee grown in the United States, primarily from the state of Hawai’i, represents less than 1% of the world’s supply. Kona coffee, grown on the western side of the island of Hawai’i, is the most recognized and highest valued Hawaiian coffee. The majority of this coffee is sun-dried after harvest and washing. Sun-dried coffee should reach 9-13% moisture within 4 to 6 days. Reducing water activity (aw) below 0.75, especially in the first week of drying, is important for preventing or limiting mold growth. The purpose of this study was to compare drying rates of Kona coffee bean batches using aw and moisture content % measurements, and evaluate factors affecting the drying time of sun-dried Kona coffee.

Ten Kona region farms were visited in the fall of 2017 to measure the drying rate of coffee bean batches and to record observations on the post-harvest handling and storage of coffee beans and the environmental conditions during drying. Daily measurements of coffee moisture level, water activity, bean layer depth and coffee temperature were recorded along with air temperature, relative humidity and cloud cover during drying for 30 batches.

Most sun-dried batches reached 13% moisture in 6 to 10 days. Initial moisture content (31.6 ± 4.3%), and drying yard characteristics varied greatly among farms. Coffee batches reached 0.75 aw within 6 days on average, but some batches required more than 10 days. Allowing airflow around drying beans and maintaining a bean layer depth of less than 5 cm appeared to improve drying rates. Drying coffee parchments within 7 days post-harvest can inhibit growth of molds that may impact quality or molds that could produce mycotoxin. Controlling the drying conditions, including raking the layered beans, and monitoring moisture content can accelerate, or improve the consistency of, batch drying time.